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Thermal and Photochemical Electrocyclic Reactions: Overview

Thermal and Photochemical Electrocyclic Reactions: Overview

Electrocyclic reactions are reversible reactions. They involve an intramolecular cyclization or ring-opening of a conjugated polyene. Shown below are two examples of electrocyclic reactions. In the first reaction, the formation of the cyclic product is favored. In contrast, in the second reaction, ring-opening is favored due to the high ring strain associated with cyclobutene formation.

Thermal Electrocyclic Reactions: Stereochemistry

Thermal Electrocyclic Reactions: Stereochemistry

The stereochemistry of electrocyclic reactions is strongly influenced by the orbital symmetry of the polyene HOMO. Under thermal conditions, the reaction proceeds via the ground-state HOMO.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.

Photochemical Electrocyclic Reactions: Stereochemistry

Photochemical Electrocyclic Reactions: Stereochemistry

The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state. Consequently, photochemical electrocyclic reactions proceed via the excited-state HOMO rather than the ground-state HOMO. Since the ground- and excited-state HOMOs have different symmetries, the stereochemical outcome of electrocyclic reactions depends on the mode of activation; i.e., thermal or photochemical.
Selection Rules: Photochemical Activation

Cycloaddition Reactions: Overview

Cycloaddition Reactions: Overview

Cycloadditions are one of the most valuable and effective synthesis routes to form cyclic compounds. These are concerted pericyclic reactions between two unsaturated compounds resulting in a cyclic product with two new σ bonds formed at the expense of π bonds. The [4 + 2] cycloaddition, known as the Diels–Alder reaction, is the most common. The other example is a [2 + 2] cycloaddition.

Pericyclic Reactions: Introduction

Pericyclic Reactions: Introduction

Pericyclic reactions are organic reactions that occur via a concerted mechanism without generating any intermediates. The reactions proceed through the movement of electrons in a closed loop to form a cyclic transition state, where rearrangement of the σ and π bonds yields specific products.
Pericyclic reactions can be classified into three categories: electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements. Electrocyclic reactions and sigmatropic...

Woodward–Hoffmann Selection Rules and Microscopic Reversibility

Woodward–Hoffmann Selection Rules and Microscopic Reversibility

Electrocyclic reactions, cycloadditions, and sigmatropic rearrangements are concerted pericyclic reactions that proceed via a cyclic transition state. These reactions are stereospecific and regioselective. The stereochemistry of the products depends on the symmetry characteristics of the interacting orbitals and the reaction conditions. Accordingly, pericyclic reactions are classified as either symmetry-allowed or symmetry-forbidden. Woodward and Hoffmann presented the selection criteria for...

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Updated: Aug 15, 2025

Novel Techniques for Observing Structural Dynamics of Photoresponsive Liquid Crystals

Novel Techniques for Observing Structural Dynamics of Photoresponsive Liquid Crystals

Published on: May 29, 2018

Recent advances in 8π electrocyclization reactions.

Lei Liu¹, Luan Du¹, Baosheng Li¹

¹School of Chemistry and Chemical Engineering, Chongqing University, Chongqing, 400044, China. libs@cqu.edu.cn.

Chemical Communications (Cambridge, England)

|January 4, 2023

Summary

This summary is machine-generated.

This review explores 8π electrocyclization, a method for building medium-sized rings crucial in pharmaceuticals. It highlights recent advances in constructing diverse ring systems, offering guidance for selective synthesis.

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Area of Science:

Organic Chemistry
Synthetic Chemistry
Medicinal Chemistry

Background:

Medium-ring systems are vital scaffolds in natural products and pharmaceuticals.
Synthesizing medium rings is challenging due to torsional strain and selectivity issues.
Electrocyclization offers a stereospecific route to construct medium rings with controlled stereocenters.

Purpose of the Study:

To review recent advancements (2000-2022) in 8π electrocyclization reactions.
To focus on the application of 8π electrocyclization in synthesizing medium-ring systems.
To provide guidance for the selective construction of medium-ring skeletons.

Main Methods:

Literature review of 8π electrocyclization reactions from 2000 to 2022.
Categorization of synthetic strategies based on target ring size and composition.
Analysis of applications in synthesizing aza-seven-membered, cycloheptene, cyclooctene, and bicyclo[4,2,0]octane frameworks.

Main Results:

Few studies exist on 8π electrocyclization, with limited applications in materials and biological contexts.
Recent research has expanded the scope of 8π electrocyclization for medium-ring synthesis.
The review covers diverse medium-ring frameworks, including nitrogen-containing heterocycles and carbocycles.

Conclusions:

8π electrocyclization is a powerful tool for stereocontrolled medium-ring construction.
This review consolidates recent progress, highlighting the potential of 8π electrocyclization.
The findings offer valuable insights for chemists aiming to synthesize complex medium-ring structures.